Fiber bed mist eliminators or separators have found widespread use in applications where very fine aerosols of under 3 microns in particle size must be separated from a gas or vapor stream (collectively referred to herein as a gas stream). The fiber beds of such separators have utilized fibers of varied diameter, ranging from as small as 5 microns or less to more than 200 hundred microns, as well as combinations thereof. The efficiency of such fiber bed separators is high, and efficiencies of 99% or higher are not uncommon. Some of the more frequent applications for fiber bed mist eliminators include removal of acid mists, such as sulfuric acid mist, in acid manufacturing, removal of plasticizer mists in the manufacture of poylvinyl chloride floor or wall coverings, and removal of water soluble solid aerosols such as emissions from ammonium nitrate prill towers.
For many applications, as where corrosive conditions and/or high temperatures are encountered, chemical grade glass fibers have been the materials of choice for fiber beds. On the other hand there are applications where the use of these fibers may not provide the desired corrosion resistance. For example, there are many industrial applications that have aqueous mist emissions which are weakly acidic or weakly alkaline. In those applications the materials of choice are synthetic polymer fibers, such as polyester fibers. The synthetic polymer fibers also have the desirable property of being softer and therefore easier to handle during the manufacture of fiber beds and in many cases the corrosion resistance is superior to glass fibers.
One problem associated with any fiber bed is that of maintaining a mechanically stable bed, that is, a bed which will retain its structural integrity without substantial shifting of fibers in the bed during aerosol collection under design operating conditions. If mechanical stability is not maintained the performance characteristics of the bed will be altered. For example, in an unstable bed the fibers can mat in localized areas of the bed making those areas more resistant to the flow of liquid or gas. However, the art has developed various ways for providing relatively stable fiber beds, such as the selection of fiber diameter, bed thickness and packing density combinations, and the use of reinforcing materials without also inducing flooding conditions within the bed. In the case of prior art fiber beds made from synthetic polymer fibers (bulk packed beds), one solution has been to maintain a high enough packing density to initially provide mechanical bed stability, followed by an appropriate heat treatment of the bed, referred to as annealing, to relieve mechanical stresses in the fiber and thereby provide a polymer fiber bed which will remain stable under operating conditions. In the annealing of synthetic polymer fiber beds the annealing temperature depends upon the chemistry and physical properties of the polymer fiber used. If, as preferred, annealing is done after the fiber is packed in the filter cage, the construction material of the filter cage must be able to withstand the annealing temperature. Typically, however, the cage is of a different material of construction than the fiber bed and is selected on the basis of corrosion considerations and cost. As a result, it often occurs that the maximum working temperature of the cage is less than the desired annealing temperature, eliminating the opportunity to utilize a desired design. As an example, polypropylene has many properties which make it a desireable material for the construction of cages and polyester fibers are the material of choice for some bulk pack fiber bed elements. However, a temperature of about 30.degree. F. is required for annealing polyester but the maximum working temperature of polypropylene is about 175.degree. F., making this combination of materials unavailable.
Other problems associated with the use of synthetic polymer fibers are that fiber lengths are limited to a maximum of about 3 inches (7.62 cm) since longer staple lengths cannot be processed in the currently available carding machines, the fibers are soft and pliable, and the finer the fiber diameter the softer the final Product. Therefore, in order to make a stable high efficiency fiber bed, relatively high packing densities are required. For polyester bulk fiber beds a bed density of greater than 8 lbs/ft.sup.3 (128 kg/m.sup.3) is typically used. As a result the bulk packing operation is difficult and expensive and also limits the choice of a material of construction for the filter cage.